(GUID Partition Table) The format used to define the hard disk partitions in computers with UEFI startup firmware. The GUID Partition Table (GPT) replaces the previous master boot record (MBR) method. While the MBR supported partitions as large as 2.2TB, GPT partitions can be up to 18 exabytes. GPT supports a "protective" MBR so that third-party MBR utilities can identify the disk correctly, as well as a "legacy" MBR if the first partition must boot as an MBR drive. See UEFI and MBR disk.